I think the best backwards-compatible calendar that we should adopt is the ISO week date calendar, commonly used in finance: forget months, every day is a week number + a day. So today is 44.7, which means week 44, day 7. This also gets you a nice almost even 13 weeks/quarter, and easy calculations (that report is due w48.5, aka Friday in 4 weeks). We used a worse version of this calendar in Intel and it was great (we callled them work weeks instead and iirc it was not iso8601 aligned).
This way, people can play with legacy dates for their birthdays, religious observances, etc but use a sane system at all other times. This already happens with Chinese/Islamic calendars anyways, Chinese new year is a date I have to look up every year, and that’s OK because the calendar is used for literally nothing else outside traditional festivals. Optimise for the common case, after all. Christmas is w52.1 this year, w52.3 the next, etc, just as CNY is 22-jan and 10-feb this and next year (but 1/1 on the Chinese calendar).
The only trouble is that some years have 52 weeks and some 53, but such is the price to pay for backwards compatibility.
This way, people can play with legacy dates for their birthdays, religious observances, etc but use a sane system at all other times. This already happens with Chinese/Islamic calendars anyways, Chinese new year is a date I have to look up every year, and that’s OK because the calendar is used for literally nothing else outside traditional festivals. Optimise for the common case, after all. Christmas is w52.1 this year, w52.3 the next, etc, just as CNY is 22-jan and 10-feb this and next year (but 1/1 on the Chinese calendar).
The only trouble is that some years have 52 weeks and some 53, but such is the price to pay for backwards compatibility.